Abstract

The 3000 and 3500 orebodies form part of the world class copper deposit atMount Isa. The Deep Copper Mine (now renamed Enterprise Mine) came intoproduction in 1993 and provided the opportunity to study the characteristics ofthese orebodies and the structural controls on copper and cobalt mineralisation.In the Enterprise Mine orebodies, chalcopyrite and silica rich breccia cores aresurrounded by a halo of recrystallised dolomitic shales and siliceous shales withlesser dolomite breccias. A distinctive talc+pyrrhotite alteration zone occcurs onthe footwall of the 3500 Orebody adjacent to a band of sub-economic Ag-Pb-Znmineralisation, which has associated phyllosilicate alteration. The Ag-Pb-Znmineralisation is unrelated to the chalcopyrite and silica-dolomite alteration. The two orebodies are situated on the west dipping limb of the Mount Isa Fold,which formed during the third period of regional deformation (D3). There are twomain structural controls that localised the copper bearing hydrothermal fluids:(1) The basement contact fault is a large scale regional fault that has been affectedby all three regional deformation periods. It separates the Urquhart Shale (orebodyhost sequence) from the older Eastern Creek Volcanics. The position of D3 relatedflexures in the basement contact fault are a major control on the localisation ofcopper mineralisation and possibly of cobalt mineralisation.(2) Bedding parallel shear zones and associated splay shear zones form ananastomosing network within the orebodies. The orientation of bedding and faults,and indicators that bedding plane slip was an important process, suggest that theshear zones formed towards the latter stages of development of the Mount Isa Foldand that the shear zone geometry strongly influenced the position of the orebodies.The bedding parallel shear zones were pathways for the hydrothermal fluids,especially in the up-dip direction. They have a strong influence on local variationsin copper distribution. The shear zones flatten in dip towards the basement contactfault and propagate from D3 flexures. The shear zone networks are mostextensively developed in the more laminated sections of the.original stratigraphy,and occur as contacts between different layers variously affected by the carbonateand silica bearing hydrothermal fluids.

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